Glass base material, optical fiber, method for manufacturing thereof, and method for determining cause of defect thereof

ABSTRACT

A glass base material, which is a base material of an optical fiber, comprising: a core; and a clad surrounding the core; wherein: a rate of change in a relative-refractive-index-difference between the core and the clad in a longitudinal direction of the glass base material is substantially 6% or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent appl. Ser. No. 10/116,744, filed Apr5, 2002, which claims priority to Japanese Patent Appl. No. 2001-108941,filed Apr. 6, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a glass base material, single modeoptical fiber, a method for manufacturing thereof, and a method fordetecting a defect thereof. More particularly, the present inventionrelates to a glass base material, single mode optical fiber, a methodfor manufacturing thereof, and a method for determining a cause ofdefect thereof by which a part of an optical fiber that causes atransmission loss can be easily specified.

2. Description of the Related Art

As one of the methods to measure a transmission loss in an opticalfiber, there is a cutback method. The cutback method inputs a lighthaving a predetermined wavelength into one end of an optical fiber andmeasures a power of light that exits from the other end of the opticalfiber. Next, an incident side of the end of the optical fiber is cut forsubstantially 2 m.

Then, a light having a predetermined wavelength is input into one end ofan optical fiber, which is about 2 m in length, and a power of lightthat exits from the other end is measured again. A difference betweenthe two powers of lights is calculated. The difference of the two powersof lights is a transmission loss occurring in the remaining opticalfiber, the power of light that traveling through which is not measured.

The cutback method can accurately measure an average transmission lossfor the whole length of the optical fiber. However, it is difficult tomeasure a distribution of a transmission loss in the longitudinaldirection of the optical fiber by the cutback method. The cutback methodhas the following disadvantage. In a case where the transmission loss ishigh, the cutback method cannot obtain the information whether thetransmission loss is high over the whole length of the optical fiber orthe transmission loss is high only on a part of the optical fiber. Also,the cutback method cannot detect the location of the part having a hightransmission loss in the optical fiber.

As another method for obtaining the information of transmission loss inthe longitudinal direction of the optical fiber, there is an OTDR(Optical Time Domain Reflectometer) method. The OTDR method measures atransmission loss in the optical fiber by inputting a pulse light havinga predetermined wavelength from one end of the optical fiber. The OTDRmethod then measures a Rayleigh-scattering-light, which is returned froma position of z of the optical fiber to one end of the optical fiber, towhich the pulse light is input. The “z” is a distance from the incidentend plane of the optical fiber. Hereinafter, theRayleigh-scattering-light is referred to as a backscattering light.

The strength of the backscattering light P(λ, Z) is calculated by thefollowing equation (1). $\begin{matrix}{{P\left( {\lambda,z} \right)} = {{P_{0} \cdot {\alpha\left( {\lambda,z} \right)} \cdot {B\left( {\lambda,z} \right)}}\exp\left\{ {{- 2}{\int_{0}^{z}{{\gamma(x)}\quad{\mathbb{d}x}}}} \right\}}} & (1)\end{matrix}$

-   P₀: strength of a propagation light at an incident end (z=0)-   α: Rayleigh-scattering-coefficient.-   B: backscattering-light-collect-coefficient-   γ: local transmission loss.

When the equation (1) is transformed using a logarithmic value and isexpressed by a dB scale, the equation (1) is transformed into anequation (2). $\begin{matrix}\begin{matrix}{{S\left( {\lambda,z} \right)} = {10\log\quad\sqrt{P\left( {\lambda,z} \right)}}} \\{= {{5\quad\log\quad{P_{0} \cdot {\alpha(\lambda)}}} + {5\log\quad{B\left( {\lambda,z} \right)}} - {10\left( {\log\quad e} \right){\int_{0}^{z}{{\gamma(x)}\quad{\mathbb{d}x}}}}}}\end{matrix} & (2)\end{matrix}$

As shown in FIG. (2), the backscattering-light-strength S (λ, z) changesaccording to the position “z” in the longitudinal direction of theoptical fiber. Here, the Rayleigh scattering coefficient α is assumed tobe substantially constant along the longitudinal direction of theoptical fiber when the transmission loss in the longitudinal directionof the optical fiber is relatively stable.

FIG. 1 shows an example of a result of typical OTDR measurement. A part,where the backscattering-light-strength S(λ, z) simply decreases,indicates that the characteristic of the transmission loss is stable.The abrupt change in the inclination of the line around z=10,000 mindicates that the abrupt increase in the transmission loss occurs atthe position z=10,000 m.

As a cause of the abrupt increase in the transmission loss, such as amacro bending loss, which occurs when the optical fiber is bent bystress applied on the optical fiber. The transmission loss may abruptlyincrease when there is a defect in the connection between two opticalfibers. The region having a high transmission loss is not desirable fortransmitting a light signal. It is necessary to re-lay or re-connect theoptical fiber in the region having a high transmission loss.

It is difficult to accurately measure the transmission loss by the OTDRmethod if only one side of the backscattering light, which is input toone side of the end of the optical fiber and returned to this side ofthe end of the optical fiber, is measured. Hereinafter, the measurementof the transmission loss by the OTDR method is referred to as OTDRmeasurement.

As made clear from the equation (1), the factor, which influences thestrength of the backscattering light, is not limited to the transmissionloss γ(z). The fluctuation in thebackscattering-light-collect-coefficient B(z) also influences thestrength of the backscattering light. Thus, the waveform of the lightthat propagates through the optical fiber fluctuates when thebackscattering-light-collect-coefficient B(z) fluctuates.

In order to measure a transmission loss accurately, the backscatteringlight is measured from both ends of the optical fiber. Therefore, onebackscattering-light-strength S₁(λ, z) is measured from one end of theoptical fiber, and another backscattering-light-strength S₂(λ, z-L) ismeasured from another end of the optical fiber. Thus, the values of thebackscattering-light-strength S₁(λ, z) and S₂(λ, z-L) shown in thefollowing equations (3) and (4) are obtained. $\begin{matrix}{{S_{1}\left( {\lambda,z} \right)} = {{5\quad\log\quad{P_{01} \cdot {\alpha(\lambda)}}} + {5\log\quad B\left( {\lambda,z} \right)} - {10\left( {\log\quad e} \right){\int_{0}^{z}{{\gamma(x)}\quad{\mathbb{d}x}}}}}} & (3) \\\begin{matrix}{{S_{2}\left( {\lambda,{z - L}} \right)} = {{{- 5}\quad\log\quad{P_{02} \cdot {\alpha(\lambda)}}} - {5\log\quad{B\left( {\lambda,z} \right)}} +}} \\{10\left( {\log\quad e} \right){\int_{0}^{z}{{\gamma(x)}\quad{\mathbb{d}x}}}}\end{matrix} & (4)\end{matrix}$

Then, a value of D(z) is obtained by the following equation (5). Asshown in equation (5), the factor of thebackscattering-light-collect-coefficient B(z) is canceled out by addingthe values of the backscattering-light-strengths S₁ (λ, z) and S₂(λ,z-L). Therefore, only the component of the transmission loss γ remainsin equation (5). $\begin{matrix}\begin{matrix}{{D(z)} = {\left\{ {{S_{1}\left( {\lambda,z} \right)} + {S_{2}\left( {\lambda,{z - L}} \right)}} \right\}/2}} \\{= {{{const}.{- 10}}\left( {\log\quad e} \right){\int_{0}^{z}{{\gamma(z)}\quad{\mathbb{d}x}}}}}\end{matrix} & (5)\end{matrix}$

If the optical fiber is in a condition where the optical fiber is woundaround a bobbin right after the manufacture of the optical fiber, it ispossible to perform OTDR measurement from both ends of the opticalfiber. However, it is extremely difficult to perform OTDR measurementfrom both ends of the optical fiber if the optical fiber is formed intocable and is laid linearly for more than 10 kilometers.

Hence, in the actual construction site, it was difficult to specify thepart, in which the transmission loss is occurred, by measuring thebackscattering-light-strength S(λ, z) from only one end of the opticalfiber. If the backscattering-light-strength S(λ, z) is measured fromonly one end, the part having a largebackscattering-light-collect-coefficient B(z) maybe mistakenlyconsidered as the part in which the transmission loss is occurred.

Thus, if there is a part having a largebackscattering-light-collect-coefficient B(z) in the optical fiber, thispart may be mistakenly considered to have a high transmission loss evenif the transmission loss is actually low in this part.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide anoptical fiber capable of easily specifying a part with a problem oftransmission loss by OTDR measurement from only one end of an opticalfiber and to provide an optical fiber base material to manufacture sucha fiber and an evaluation method thereof.

According to a first aspect of the present invention, a glass basematerial, which is a base material of an optical fiber, comprises acore; and a clad surrounding the core; wherein a rate of change in arelative-refractive-index-difference between the core and the clad in alongitudinal direction of the glass base material is substantially 6% orless.

According to a second aspect of the present invention, an optical fibercomprises a core; and a clad surrounding the core; wherein a rate ofchange in a relative-refractive-index-difference between the core andthe clad in a longitudinal direction of the optical fiber issubstantially 6% or less.

According to a third aspect of the present invention, an optical fibercomprises a core; and a clad surrounding the core; wherein an absolutevalue of an amount of change in abackscattering-light-collect-coefficient |I(λ, z)| in a longitudinaldirection of the optical fiber is substantially 0.1 dB or less.

The optical fiber may have a region, which satisfies a relation of|I(λ₁, z)|<|I(λ₂, z)|≦0.1 dB at least in a part of a longitudinaldirection of the optical fiber where I(λ₁, z) and I(λ₂, z) are theamounts of change in a backscattering-light-collect-coefficient |I(λ,z)|for two wavelengths λ₁ and λ₂, where λ₁<λ₂.

The optical fiber may have a region, which satisfies a relation of I(λ₁,z)=0.8×I(λ₂, z), at least in a part of longitudinal direction of theoptical fiber when the two wavelengths λ₁ and λ₂ are substantially 1310nm and 1550 nm, respectively.

According to a fourth aspect of the present invention, a glass basematerial, which is a base material of an optical fiber, comprises acore; and a clad surrounding the core; wherein a rate of change in adiameter of the core in a longitudinal direction of the glass basematerial is substantially 7% or less.

According to a fifth aspect of the present invention, an optical fibercomprises a core; and a clad surrounding the core; wherein a rate ofchange in a diameter of the core in a longitudinal direction of theoptical fiber is substantially 7% or less.

According to a sixth aspect of the present invention, an optical fibercomprises a core; and a clad surrounding the core; wherein an amount ofchange in a backscattering-light-collect-coefficient in a longitudinaldirection of the optical fiber is substantially 0.1 dB or less.

The optical fiber may have a region, which satisfies a relation of|I(λ₂, z)|<|I(λ₁, z)|≦0.1 dB at least in a part of a longitudinaldirection of the optical fiber where I(λ₁, z) and I(λ₂, z) are theamounts of change in a backscattering-light-collect-coefficient I(λ, z)for two wavelengths λ₁ and λ₂, where λ₁<λ₂.

The optical fiber may have a region, which satisfies a relation of I(λ₁,z)=2×I(λ₂, z), at least in a part of a longitudinal direction of theoptical fiber when the two wavelengths λ₁ and λ₂ are substantially 1310nm and 1550 nm, respectively.

According to a seventh aspect of the present invention, a method fordetermining a cause of defect in an unused optical fiber comprisesmeasuring backscattering-light-strengths S(λ₁, z) and S(λ₂, z) of theoptical fiber for two wavelengths λ₁ and λ₂, where λ₁<λ₂ from both endsof the optical fiber; calculating an amount of change in abackscattering-light-collect-coefficient I(λ₂, z) and I(λ₁, z) for thetwo wavelength λ₁ and λ₂; comparing the amount of change in abackscattering-light-collect-coefficient I(λ₁, z) and I(λ₂, z) for thetwo wavelengths λ₁ and λ₂ to examine whether the amount of change in abackscattering-light-collect-coefficient I(λ₁, z) and I(λ₂, z) satisfy apredetermined relationship; and determining the cause of defect in theoptical fiber according to the comparison.

The comparing may examine whether the amount of change in abackscattering-light-collect-coefficient I(λ₁, z) and I(λ₂, z) satisfy arelationship of |I(λ₁, z)|>0.1 dB, |I(λ₂, z)|>0.1 dB, and |I(λ₁,z)|<|I(λ₂, z)| in the optical fiber; and the determining may determinethat the cause of defect is in a rate of change in arelative-refractive-index-difference of the optical fiber in alongitudinal direction when the relationships are satisfied.

The comparing may examine whether the amount of change in abackscattering-light-collect-coefficient I(λ₂, z) and I(λ₁, z) satisfy arelationships of |I(λ₁, z)|>0.1 dB, |I(λ₂, z)|>0.1 dB, and |I(λ₁,z)>|I(λ₂, z)| in the optical fiber; and the determining may determinethat the cause of defect is in a rate of change in a core diameter ofthe optical fiber in longitudinal direction when the relationships aresatisfied.

According to a eighth aspect of the present invention, a method formanufacturing a glass base material, which is a base material of anoptical fiber, comprises forming a clad around a core by accumulatingglass particles on the core to form a porous-glass-base-material;sintering and dehydrating the porous-glass-base-material to form theglass base material; measuring a rate of change in arelative-refractive-index-difference between the core and the clad in alongitudinal direction of the glass base material; and removing theglass base material, the rate of change in arelative-refractive-index-difference of which is more than substantially6%.

According to a ninth aspect of the present invention, a method formanufacturing an optical fiber comprises drawing a glass base material,which is a base material of the optical fiber having a core and a cladthat surrounds the core, to form the optical fiber; measuring a rate ofchange in a relative-refractive-index-difference between the core andthe clad in a longitudinal direction of the optical fiber; and removinga part of the optical fiber, the rate of change in arelative-refractive-index-difference of which is more than substantially6%.

According to a tenth aspect of the present invention, a method formanufacturing a glass base material, which is a base material of anoptical fiber, comprises forming a clad around a core by accumulatingglass particles on the core to form a porous-glass-base-material;sintering and dehydrating the porous-glass-base-material to form theglass base material; measuring a rate of change in a diameter of thecore in a longitudinal direction of the glass base material; andremoving the glass base material, the rate of change in the diameter ofthe core of which is more than substantially 7%.

According to a eleventh aspect of the present invention, a method formanufacturing an optical fiber comprises drawing a glass base material,which is a base material of the optical fiber having a core and a cladthat surrounds the core, to form the optical fiber; measuring a rate ofchange in a core diameter in a longitudinal direction of the opticalfiber; and removing a part of the optical fiber, the rate of change inthe core diameter of which is more than substantially 7%.

According to a twelveth aspect of the present invention, a method formanufacturing an optical fiber comprises drawing a glass base material,which is a base material of the optical fiber having a core and a cladthat surrounds the core, to form the optical fiber; measuring an amountof change in a backscattering-light-collect-coefficient in alongitudinal direction of the optical fiber; removing the optical fiber,in which an absolute value of the amount of change in abackscattering-light-collect-coefficient |I(λ, z) is more thansubstantially 0.1 dB.

The summary of the invention does not necessarily describe all necessaryfeatures of the present invention. The present invention may also be asub-combination of the features described above. The above and otherfeatures and advantages of the present invention will become moreapparent from the following description of the embodiments taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a result of typical OTDR measurement.

FIGS. 2 shows the detailed procedure of the OTDR measurement method.

FIGS. 3A-3C show the change of the backscattering-light-strength S(λ, z)along the longitudinal direction of the optical fiber.

FIG. 3D shows a value of D(z), which changes in a longitudinal directionof the optical fiber.

FIG. 4A and 4B show the backscattering-light-strength S (λ, z), which ismeasured from one end of the optical fiber.

FIG. 4C shows a D(z) calculated by the equation (11).

FIG. 4D shows an amount of change in thebackscattering-light-collect-coefficient I(λ, z).

FIG. 4E shows an MFD measured along the longitudinal direction of theoptical fiber.

FIG. 5 shows amounts of change in thebackscattering-light-collect-coefficients I(λ₁, z) and I(λ₂, z).

FIG. 6 shows relative values of I(λ₁, z) and I(λ₂, z).

FIG. 7 shows the calculated relative value of I(λ₁, z) and I(λ₂, z).

FIG. 8 shows measured values of I(λ₁, z) and I(λ₁, z).

FIG. 9 shows calculated relative values of I(λ₁, z) and I(λ₂, z).

FIG. 10 shows a change of absolute or relative value of I(λ₁, z) andI(λ₂, z).

FIG. 11 shows a flowchart of the manufacturing process of the opticalfiber using the present embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described based on preferred embodiments,which do not intend to limit the scope of the present invention, butrather to exemplify the invention. All of the features and thecombinations thereof described in the embodiments are not necessarilyessential to the invention.

As one of the factors of the problem on the transmission loss of theoptical fiber, there is a macro bending loss. This macro bending loss isa phenomenon, in which a part of the light that propagates in theoptical fiber escapes from the optical fiber when the optical fiber isbent with an order between 1 mm and 100 mm. This phenomenon of the macrobending loss is observed as a transmission loss.

The degree of the transmission loss caused by the macro bending loss isdifferent according to the condition of the distribution of therefractive index of the optical fiber or the magnitude of the bending.

Generally, there is a tendency such that the more the dend radius of thebent section of the optical fiber decreases, the more the degree of thetransmission loss of the macro bending loss increases. Also, the morethe length of the bent section increases, the more the degree of thetransmission loss of the macro bending loss increases. Furthermore, themore the cutoff wavelength decreases, the more the degree of thetransmission loss of the macro bending loss increases. Furthermore, themore the mode field diameter increases, the more the degree of thetransmission loss of the macro bending loss increases. Also, the morethe transmission wavelength increases, the more the degree of thetransmission loss of the macro bending loss increases.

It is required that the micro bending loss against the bending of adiameter of 32 mmφ for one lap should be 1 dB or less according to G.652of ITU-T (Telecommunication standardization sector of internationaltelecommunication union), which is an international standard.

Another factor of the problem on the transmission loss of the opticalfiber is a connection loss. The connection loss occurs when theoverlapped two power distribution of the light in two optical fibers arenot matched with each other at the connection point of two opticalfibers. One of the causes of the connection loss is a gap createdbetween two-core axes of two optical fibers at the connection point oftwo optical fibers.

For example, in a case of a typical single mode optical fiber having astep type refractive index distribution and a mode field diameter (MFD)of 9.2 μm, the connection loss of 0.2 dB occurs when two core axes oftwo optical fibers are connected with a gap of 1 μm.

In the present invention, the optical fiber itself is made so that apart of the optical fiber having a transmission loss more thansubstantially 0.1 dB can be easily detected by measuring the incidentallight of one end using the OTDR measurement method.

FIGS. 2 to 4E show the detailed procedure of the OTDR measurementmethod. In FIGS. 2 to 4B, the vertical axis indicates abackscattering-light-strength S(λ, z), and the horizontal axis indicatesa distance “z” from an incident end face of the optical fiber. “L” is alength of the optical fiber. In FIG. 4C, the vertical axis indicates aD(z) calculated by the equation (11). The vertical axis of FIG. 4Dindicates I(λ, z), which is an amount of change in abackscattering-light-collect-coefficient B(z). The vertical axis of FIG.4E indicates a value of MFD.

As shown in FIG. 2, the backscattering-light-strength S(λ, z) decreaseslinearly in the longitudinal direction of the optical fiber when thetransmission loss is constant over the whole length of the opticalfiber.

FIGS. 3A and 3B show the change of the backscattering-light-strengthS(λ, z) along the longitudinal direction of the optical fiber when thereis a part that has high transmission loss in the optical fiber. Here,FIG. 3A shows the backscattering-light-strength S(λ, z), which ismeasured from one end of the optical fiber. On the other hand, FIG. 3Bshows the backscattering-light-strength S(λ, z), which is measured fromanother end of the optical fiber.

FIG. 3C shows the backscattering-light-strength S(λ, z-L), which isobtained by reversing FIG. 3B in the horizontal direction. For example,the distance “z” of “0” is on the left hand side in FIG. 3B, and itbecomes the right hand side in FIG. 3C. Also, the distance “z” of “L” ison the right hand side in FIG. 3B, and it becomes the left hand side inFIG. 3C.

FIG. 3D shows a value of D(z), which changes in a longitudinal directionof the optical fiber. The value of D(z) is calculated by theabove-mentioned equation (5) from the values of thebackscattering-light-strengths S(λ,z) and S(λ,z-L) shown in FIGS. 3A and3C. In FIG. 3D, the inclination of D(z) increases greatly at the partmarked by “X”. Therefore, the part having a high transmission loss,which is marked by “X”, can be detected.

FIGS. 4A and 4B show the backscattering-light-strengths S(λ, z) along alongitudinal direction of the optical fiber when there is a part, inwhich the backscattering-light-collect-coefficient B changes. Here, FIG.4A shows the backscattering-light-strength S(λ, z), which is measuredfrom one end of the optical fiber. FIG. 4B shows thebackscattering-light-strength S(λ, z), which is measured from anotherend of the optical fiber.

In FIGS. 4A and 4B, the inclination of the change in thebackscattering-light-strength S(λ, z) changes at the part marked by “Y”.Furthermore, the direction of the inclination of the change in thebackscattering-light-strength S(λ, z) is different between FIGS. 4A and4B. For example, in FIG. 4A, the backscattering-light-strength S(λ, z)decreases at the part marked by “Y”. In contrast, in FIG. 4B,backscattering-light-strength S(λ, z) increases at the part marked by“Y”.

FIG. 4C shows the change in D(z) along the longitudinal direction of theoptical fiber. FIG. 4C is calculated from the values in FIGS. 4A and 4Busing the same method as explained in FIGS. 3A to 3D. As shown in FIG.4C, the change in the backscattering-light-strength S(λ, z) at the partmarked by “Y” is canceled out. Thus, the backscattering-light-strengthS(λ,z) decreases linearly for the whole length of the optical fiber.Therefore, as clearly shown in FIG. 4C, the transmission loss is notchanged at the part marked by “Y”. Thus, it is understood that the causeof the change in the inclination of the backscattering-light-strengthS(λ, z) is not a transmission loss but a change in thebackscattering-light-collect-coefficient B.

FIG. 4D shows an amount of change in thebackscattering-light-collect-coefficient I(λ, z). The amount of changein the backscattering-light-collect-coefficient I(λ, z) is calculated bythe following equation (6). $\begin{matrix}\begin{matrix}{{I\left( {\lambda,z} \right)} = {\left\{ {{S_{1}\left( {\lambda,z} \right)} - {S_{2}\left( {\lambda,{z - L}} \right)}} \right\}/2}} \\{= {{{const}.{+ 5}}\quad\log\quad{B\left( {\lambda,z} \right)}}}\end{matrix} & (6)\end{matrix}$

Also, the backscattering-light-collect-coefficient B is calculated bythe following equation (7). $\begin{matrix}{B = {\left\{ {3/\left\lbrack {4k^{2}a^{2}\Delta\quad n^{2}} \right\rbrack} \right\} \times \left\{ {\left\lbrack {\int_{0}^{\infty}{{\varphi^{4}(r)}r\quad{\mathbb{d}r}}} \right\rbrack/\left\lbrack {\int_{0}^{\infty}{{\varphi^{2}(r)}r\quad{\mathbb{d}r}}} \right\rbrack^{2}} \right\}}} & (7)\end{matrix}$

-   k: constant determined by a wavelength of propagated light-   a: core diameter-   Δn: refractive index difference of an optical fiber-   φ: electrical field distribution of propagated light-   r: distance or radius from a center of an optical fiber

The cause of the fluctuation in thebackscattering-light-collect-coefficient B is the fluctuation in modefield diameter (MFD). There is a relationship of B∝[MFD]⁻² between thebackscattering-light-collect-coefficient B and the MFD.

FIG. 4E shows an MFD measured along the longitudinal direction of theoptical fiber. An MFD is measured by cutting an optical fiber along alongitudinal direction. It can be understood from FIGS. 4D and 4E thatthe cause of the fluctuation in the I(λ, z) is the fluctuation in theMFD value because when the MFD value abruptly changes at the part markedby “Y”, the value of the I(λ, z) also abruptly changes at the partmarked by “Y”.

From a different point of view, if the fluctuation of the MFD in thelongitudinal direction of the optical fiber is reduced, the fluctuationin the backscattering-light-collect-coefficient B in the longitudinaldirection of the optical fiber is also reduced. Thus, by reducing thefluctuation in the MFD in the optical fiber to reduce the fluctuation inthe backscattering-light-collect-coefficient B, the transmission loss inthe optical fiber can be detected by measuring thebackscattering-light-strength S(λ, z) from only one end of the opticalfiber. Therefore, it is possible to detect the part having a hightransmission loss in the optical fiber by the OTDR measurement thatmeasures the backscattering light from only one end of the opticalfiber.

There are two causes of the fluctuation in MFD. One of the causes is afluctuation in a relative-refractive-index-difference Δn. The Δn iscalculated by the following equation.Δn={(n ₁ −n ₂)/n ₁}×100[%]  (8)

-   n₁: refractive index of core-   n₂: refractive index of clad

Another cause is a fluctuation in a diameter of a core of an opticalfiber.

A glass base material having a diameter between 10-200 millimeters wasmanufactured. Then, an optical fiber having a diameter of 120 μm ismanufactured by heating and softening the glass base material anddrawing the softened glass base material.

The refractive index distribution of the glass base material is adjustedso as to have a similar shape with that of the optical fiber. Thefluctuation of each parameter explained above, such asrelative-refractive-index-difference Δn and a diameter of a core of anoptical fiber, may occur in the phase of manufacturing the glass basematerial. The fluctuation of each parameter explained above may alsooccur in the phase of drawing the glass base material to be an opticalfiber.

That is, a the refractive index or thee core diameter of a glass basematerial fluctuates in the phase of manufacturing the glass basematerial, the refractive index or the core diameter of the optical fibermanufactured from the glass base material also fluctuates almost thesame as the glass base material. Even if the fluctuation of therefractive index or the core diameter is reduced in the phase ofmanufacturing the glass base material, the refractive index or the corediameter of the optical fiber may fluctuate if a speed of cooling anoptical fiber during the drawing process fluctuates.

If the speed of cooling an optical fiber fluctuates during the drawingprocess, the condition of a residual stress inside the optical fiberchanges. Thus, the refractive index distribution of the drawn opticalfiber also changes. Furthermore, if the diameter of the optical fiberfluctuates during the drawing process, the core diameter of the drawnoptical fiber also changes. It is very difficult to cancel thefluctuation of the refractive index distribution or the core diameter ofthe optical fiber by controlling the drawing process of the opticalfiber if the refractive index distribution or the core diameter of theglass base material fluctuates.

Therefore, in order to manufacture an optical fiber having smallfluctuation in the above-mentioned parameters, it is necessary tocontrol the fluctuation of the above-mentioned parameters to a minimumduring both the phase of manufacturing the glass base material anddrawing the glass base material into the optical fiber.

Furthermore, if a fluctuation of the above-mentioned parameters isrecognized in the glass base material by the test, the glass basematerial is removed from the manufacturing process so that the glassbase material having stable values of the above-mentioned parameters isshipped. Furthermore, if a fluctuation of the above-mentioned parametersis recognized in the part of the optical fiber by the test, thefluctuated part of the optical fiber is removed in order to take outonly the part of the optical fiber having a stable value of theabove-mentioned parameters.

Therefore, how an amount of fluctuation in the above-mentionedparameters, such as refractive index distribution or the core diameter,influences on the fluctuation in the value measured by the OTDRmeasurement method, such as backscattering-light-strength S(λ, z), wasresearched. Especially, how the fluctuation in the refractive indexdistribution or the fluctuation in the core diameter influences on thefluctuation in the backscattering-light-collect-coefficient B wasresearched.

First, a glass base material was formed by a VAD method. Therelative-refractive-index-difference Δn of the core of the manufacturedglass base material was fluctuated from a reference value ofrelative-refractive-index-difference Δn=0.35% in a longitudinaldirection. This glass base material was drawn into an optical fiberhaving a core diameter of 8 μm. The OTDR measurement is performed onthis optical fiber.

FIGS. 5 to 7 show a result of this OTDR measurement on the opticalfiber, which is drawn from the glass base material having a fluctuatedrelative-refractive-index-difference.

FIG. 5 shows amounts of change in thebackscattering-light-collect-coefficients I(λ₁, z) and I(λ₂, z) for twowavelengths λ₁=1310 nm and λ₂=1550 nm calculated using theabove-mentioned equation (6). The values of I(λ₁, z) and I(λ₂, z) arecalculated for each position of “z” along a longitudinal direction ofthe optical fiber. Then, the values of I(λ₁, z) and I(λ₂, z) for eachposition of “z” are plotted on FIG. 5.

Next, the value of the electrical field distribution φ of the opticalfiber, which had a core diameter of 8 μm, was calculated by computersimulation. During the calculation of the electrical field distributionφ, the relative-refractive-index-difference Δn fluctuated slightly froma reference value of the relative-refractive-index-difference Δn=0.35%.

Then, the backscattering-light-collect-coefficient B was calculatedusing the above-mentioned equation (7). Next, the value of I(λ, z) iscalculated using the equation (6). Finally, an absolute or relativevalue of I(λ, z) is calculated by calculating a difference between theI(λ, z), which is calculated using the fluctuatedrelative-refractive-index-difference Δn, and the reference I(λ, z),which is calculated using the reference value of therelative-refractive-index-difference Δn=0.35%.

FIG. 6 shows relative values of I(λ₁, z) and I(λ₂, z) for λ₁=1310 nm andλ₂=1550 nm calculated by the computer simulation. It is clear from FIGS.5 and 6 that the inclination of the simulated relative value of I(λ, z)is about the same as the inclination of the measured relative value ofI(λ, z).

FIG. 7 shows the change of the relative value of I(λ₁, z) and I(λ₂, z)with the rate of change in the relative-refractive-index-difference Δncalculated by the computer simulation. The vertical axis shows anabsolute or relative value of I(λ₁, z) and I(λ₂, z). The horizontal axisshows a rate of change in the relative-refractive-index-difference Δnfrom the reference value of the relative-refractive-index-differenceΔn=0.35%. The solid line shows the relative value of I(λ₁, z) forλ₁=1310 nm, and the broken line shows the relative value of I(λ₂, z) forλ₂=1550 nm.

It can be recognized from FIG. 7 that if the rate of change in therelative-refractive-index-difference Δn is substantially 1.06 or less,that is, substantially 6% or less, the absolute or relative value ofI(λ₁, z) and I(λ₂, z) becomes substantially 0.1 dB or less. If theabsolute or relative value of I(λ₁, z) is about 0.1 dB or less, theconnection loss can be detected by the OTDR measurement from only oneend of an optical fiber. When the relative-refractive-index-differenceΔn fluctuates and the core diameter does not fluctuate, |I(λ₂)| becomeslarger than |I(λ₁)|. It can also be recognized from FIG. 7 that there isa relation between I(λ₁) and I(λ₂) of I(λ₁)≈0.8×I(λ₂) when λ₁ is 1310 nmand λ₂ is 1550 nm.

Therefore, the glass base material or the optical fiber is manufacturedsuch that the rate of change in the relative-refractive-index-differenceΔn is substantially 6% or less, then the transmission loss in theoptical fiber can be detected by the OTDR measurement from only one endof an optical fiber. Furthermore, the cause of the transmission loss canbe detected by comparing the magnitude of |I(λ₁)| and |I(λ₂)|.

FIGS. 8 to 10 show the result of this OTDR measurement performed on theoptical fiber manufactured from the glass base material. This glass basematerial was manufactured such that the outside diameter of the glassbase material was fluctuated by the surface treatment using hydrofluoricacid. Then, the glass base material was drawn to have an outsidediameter of 125 μm. Then, the optical fiber having the core diameter,which was fluctuated from a central value of 8 μm, was obtained. TheOTDR measurement was performed on this optical fiber.

FIG. 8 shows measured values of I(λ₁, z) and I(λ₁, z). Similar to FIG.5, the amount of change in the backscattering-light-collect-coefficientI(λ, z) is measured for the two wavelengths λ₁=1310 nm and λ₂=1550 nmvarying the position “z” along a longitudinal direction of the opticalfiber. Then, the measured values of I(λ₁, z) and I(λ₁, z)are plotted asin FIG. 8.

Next, the value of the electrical field distribution φ of the opticalfiber, which had a core diameter of 8 μm, was calculated by the computersimulation. During the calculation of the electrical field distributionφ, the relative-refractive-index-difference Δn was slightly fluctuatedfrom a reference value of the relative-refractive-index-differenceΔn=0.35%.

Then, the backscattering-light-collect-coefficient B was calculatedusing the above-mentioned equation (7). Next, the value of I(λ, z) iscalculated using the equation (6). Finally, an absolute or relativevalue of I(λ, z) is calculated by calculating a difference between theI(λ, z), which is calculated using the fluctuated core diameter, and thereference I(λ, z), which is calculated using the core diameter of 8 μm.

FIG. 9 shows relative values of I(λ₁, z) and I(λ₂, z) for λ₁=1310 nm andλ₂=1550 nm calculated by the computer simulation. It is clear from FIGS.8 and 9 that the inclination of the simulated relative value of I(λ, z)shown in FIG. 9 is about the same as the inclination of the measuredrelative value of I(λ, z) shown in FIG. 8.

FIG. 10 shows a change of absolute or relative value of I(λ₁, z) andI(λ₂, z) with the rate of change in the core diameter calculated by thecomputer simulation. The vertical axis shows an absolute or relativevalue of I(λ₁, z) and I(λ₂, z). The horizontal axis shows a rate ofchange in the core diameter from the reference value of the corediameter of 8 μm. The solid line shows the change of relative value ofI(λ₁, z) for λ₁=1310 nm, and the broken line shows the change ofrelative value of I(λ₂, z) for λ₂=1550 nm.

It can be recognized from FIG. 10 that if the rate of change in the corediameter is about 1.07 or less, that is, substantially 7% or less, theabsolute or relative value of |I(λ₁, z)| and |I(λ₂, z)| becomesubstantially 0.1 dB or less. When therelative-refractive-index-difference Δn does not fluctuate, and the corediameter fluctuates, |I(λ₁)| becomes larger than |I(λ₂)|. It can also berecognized from FIG. 10 that there is a relation between I(λ₁) and I(λ₂)such that I(λ₁)≈2×I(λ₂) when λ₁ is 1310 nm and λ₂ is 1550 nm.

Therefore, the glass base material or the optical fiber is manufacturedsuch that the rate of change in the core diameter is substantially 7% orless. Then, the transmission loss in the optical fiber can be detectedby the OTDR measurement from only one end of an optical fiber.Furthermore, the cause of the transmission loss can be detected bycomparing the magnitude of the absolute or relative values of |I(λ₁, z)|and |I(λ₂, z)|. For example, if |I(λ₂)| is larger than |I(λ₁)|, thecause of the fluctuation in B is the fluctuation in therelative-refractive-index-difference Δn. Also, if |I(λ₁)| is larger than|I(λ₂)|, the cause of the fluctuation in B is the fluctuation in thecore diameter.

FIG. 11 shows a flowchart of the manufacturing process of the opticalfiber using the present embodiment. First, a porous-glass-base-materialis formed by forming a clad around a core by accumulating glassparticles on the core (S200). Next, a glass base material is formed bysintering and dehydrating the porous-glass-base-material (S202).

Then, the parameters of the glass base material are measured (S204). Asexamples of the parameters, there is a rate of change in arelative-refractive-index-difference in a longitudinal direction of theglass base material and the rate of change in the core diameter in alongitudinal direction of the glass base material.

Then, the glass base material that has the defect part, in which therate of change in a relative-refractive-index-difference is more thansubstantially 6%, is removed from the manufacturing process of theoptical fiber. Also, the glass base material that has the part, in whichthe rate of change in the core diameter is more than substantially 7%,is removed from the manufacturing process of the optical fiber (S206).Next, the glass base material is drawn into an optical fiber (S210).

Then, the parameters of the optical fiber are measured (S212). Asexamples of the parameters, there is a rate of change in arelative-refractive-index-difference in a longitudinal direction of theoptical fiber and the rate of change in the core diameter in alongitudinal direction of the optical fiber.

The cause of defect in an optical fiber can be determined as following.First, the backscattering-light-strengths S(λ₁, z) and S(λ₂, z) of theoptical fiber for two wavelengths λ₁ and λ₂, where λ₁<λ₂ are measuredfrom both ends of the optical fiber. Then, an amount of change in thebackscattering-light-collect-coefficients I(λ₂, z) and I(λ₁, z) for thetwo wavelengths λ₁ and λ₂ are calculated. Then, the amount of change inthe backscattering-light-collect-coefficients I(λ₁, z) and I(λ₂, z) forthe two wavelengths λ₁ and λ₂ are compared to examine whether the amountof change in the backscattering-light-collect-coefficients I(λ₁, z) andI(λ₂, z) satisfy a predetermined relationship.

For example, in this comparing process, whether the amount of change inthe backscattering-light-collect-coefficients I(λ₁, z) and I(λ₂, z)satisfy a relationships of |I(λ₁, z)|>0.1 dB, |I(λ₂, z)|>0.1 dB, and|I(λ₁, z)|<|I(λ₂, z)| in the optical fiber is examined. Then, it isdetermined that the cause of defect is in a rate of change in arelative-refractive-index-difference of the optical fiber in alongitudinal direction when the relationships of |I(λ₁, z)|>0.1 dB and|I(λ₂, z)|>0.1 dB, and |I(λ₁, z)|<|I(λ₂, z)| are satisfied.

Furthermore, in this comparing process, whether the amount of change inthe backscattering-light-collect-coefficients I(λ₂, z) and I(λ₁, z)satisfy a relationships of |I(λ₁, z)|>0.1 dB, |I(λ₂, z)|>0.1 dB, and|I(λ₁, z)|>|I(λ₂, z)| in the optical fiber is examined. Then, it isdetermined that the cause of defect is in a rate of change in a corediameter in longitudinal direction of the optical fiber when therelationship of |I(λ₁, z)|>0.1 dB, |I(λ₂, z)|>0.1 dB, and |I(λ₁,z)|>|I(λ₂, z)| are satisfied.

Then, the defect part of the optical fiber, in which the rate of changein a relative-refractive-index-difference in a longitudinal direction ofthe optical fiber is more than substantially 6%, is removed from themanufacturing process of the optical fiber. Also, the optical fiber thathas the part, in which the rate of change in the core diameter is morethan substantially 7%, is removed from the manufacturing process of theoptical fiber (S214).

Therefore, according to the above-explained process, an optical fiberhaving an absolute or relative value of |I(λ₁, z)|, which issubstantially 0.1 dB or less, can be obtained. Therefore, it becomespossible to specify the part, in which the transmission loss isoccurred, by measuring the backscattering-light-strength S(λ, z) fromonly one end of the optical fiber.

Although the present invention has been described by way of exemplaryembodiments, it should be understood that many changes and substitutionsmay be made by those skilled in the art without departing from thespirit and the scope of the present invention which is defined only bythe appended claims.

1. A method for determining a cause of defect in an unused optical fibercomprising: measuring, from both ends of said optical fiber,backscattering-light-strengths S(λ₁,z) and S(λ₂,z) of said optical fiberfor two wavelengths λ₁ and λ₂, where λ₁<λ₂; calculating an amount ofchange in a backscattering-light-collect-coefficient I(λ₁,z) and I(λ₂,z) for said two wavelengths λ₁ and λ₂; comparing said amount of change ina backscattering-light-collect-coefficient I(λ₁,z) and I(λ₂,z) for saidtwo wavelengths λ₁ and λ₂ to determine whether said amount of changesatisfies a predetermined relationship; and determining said cause ofdefect in said optical fiber according to said comparison.
 2. A methodas claimed in claim 1, wherein: said comparing comprises determiningwhether said amount of change in abackscattering-light-collect-coefficient I(λ₁,z) and I(λ₂,z) satisfiesrelationships of |I(λ₁,z)|>0.1 dB, |I(λ₂,z)|>0.1 dB, and|I(λ₁,z)|<|I(λ₂,z)| in said optical fiber; and said determiningcomprises concluding that said cause of defect is in a rate of change ina relative-refractive-index-difference of said optical fiber in alongitudinal direction when said relationships are satisfied.
 3. Amethod as claimed in claim 1, wherein: said comparing comprisesdetermining whether said amount of change in abackscattering-light-collect-coefficient I(λ₂,z) and I(λ₁,z) satisfiesrelationships of |I(λ₁,z)|>0.1 dB, |I(λ₂,z)|>0.1, and|I(λ₁,z)|>|I(λ₂,z)| in said optical fiber; and said determiningcomprises concluding that said cause of defect is in a rate of change ina core diameter of said optical fiber in a longitudinal direction whensaid relationships are satisfied.